Metasurfaces open up unprecedented potential for wave engineering using subwavelength sheets. However, a severe limitation of current acoustic metasurfaces is their poor reconfigurability to achieve distinct functions on demand. Here a programmable acoustic metasurface that contains an array of tunable subwavelength unit cells to break the limitation and realize versatile two-dimensional wave manipulation functions is reported. Each unit cell of the metasurface is composed of a straight channel and five shunted Helmholtz resonators, whose effective mass can be tuned by a robust fluidic system. The phase and amplitude of acoustic waves transmitting through each unit cell can be modulated dynamically and continuously. Based on such mechanism, the metasurface is able to achieve versatile wave manipulation functions, by engineering the phase and amplitude of transmission waves in the subwavelength scale. Through acoustic field scanning experiments, multiple wave manipulation functions, including steering acoustic waves, engineering acoustic beams, and switching on/off acoustic energy flow by using one design of metasurface are visually demonstrated. This work extends the metasurface research and holds great potential for a wide range of applications including acoustic imaging, communication, levitation, and tweezers.

A carbon nanotube reinforced polypyrrole nanowire network was constructed by in situ polymerization of pyrrole in the presence of carbon nanotubes using cetyltrimethylammonium bromide micelles as a soft template. Carbon nanotubes as a reinforcer were embedded into a network of polypyrrole nanowires, thus retaining in the latter a complete network. The resulting network possessed a specific surface area of 112.1 m 2 g À1 and a rough porous structure. The embedding of carbon nanotubes decreased the charge transfer resistance in the polypyrrole nanowires and allowed easy access and rapid diffusion of ions/electrons. When applied as a capacitive electrode, a specific capacitance of 183.2 F g À1 was observed at a current density of 8 A g À1 . The specific capacitance retention was 85% after 1000 cycles at 1 A g À1 . An asymmetric supercapacitor was fabricated using the network as a positive electrode and active carbon as a negative electrode, and when operated at a maximum voltage of 1.5 V, had a high energy density (15.1 W h kg À1 at 3000 W kg À1 ). A long-term cycling test of the asymmetric supercapacitor at a current density of 1 A g À1 displayed a capacitance retention of 72% even after 3000 cycles of charge and discharge.

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We report on a new optics design for an optical coherence tomography (OCT) balloon imaging catheter. The design involves a miniature compound gradient-index (GRIN) rod lens, which consists of a fiber optic mode-field reducer and relay rod lenses to achieve predictable high lateral resolution at a desired large working distance. The compound lens design significantly simplifies the engineering process for an OCT catheter and enables 3-D full circumferential cross sectional imaging of large luminal organs such as human esophagus. An as-designed OCT catheter is developed and demonstrated for real-time in vivo swine esophagus imaging in a 3-D spiral fashion. Keywordsoptical coherence tomography; endomicroscopy; balloon catheter; internal organ imaging Optical coherence tomography (OCT) is a rapidly evolving noninvasive imaging technology that provides high-resolution cross sectional images of tissue microanatomy. 1 While initially applied to eye imaging, the development of miniature fiber optic catheters/endoscopes has enabled high-resolution OCT imaging of internal luminal organs in vivo. Most of the OCT catheters developed so far have a small diameter (e.g., ∼1 to 2 mm), a high lateral resolution (e.g., 15 to 40 µm), and a short working distance (e.g., 1 to 4 mm). 2-6 These catheters are well suited for imaging small lumens or for imaging only a small sector of large lumens when the probes are in direct contact with the tissue surface. The short working distance unfortunately precludes full circumferential imaging of large luminal organs such as human esophagus (which has a diameter of ∼ 18 to 25 mm when the natural esophageal folds are flattened e.g., by balloon inflation). There is an increasing clinical need for systematic imaging assessment of the entire esophagus for Barrett's esophagus surveillance and early cancer detection, 7,8 inspiring strong interest in developing a new type of OCT catheter-a balloon imaging catheter, which basically integrates a miniature OCT imaging probe within the inner lumen of a doublelumen balloon catheter with the balloon (when inflated) to flatten the natural folds of the esophagus. The concept of such an OCT balloon imaging catheter was introduced in 2000 and was recently demonstrated for imaging the entire esophagus. 9,10 A major challenge with the OCT imaging probe is achieving a small focused spot size (i.e., high transverse resolution) at a large working distance (e.g., 9 to 12 mm) while keeping the optical components small (e.g., NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript of a diameter 1 to 2 mm), so that the imaging probe would be able to pass through a small inner balloon tubing and shaft, while the entire balloon catheter can be delivered into the esophagus through a 2.8-to 3.4-mm accessory port of a standard gastrointestinal (GI) endoscope. The systematic OCT imaging of the entire esophagus can then be integrated with a routine endoscopy procedure.We present a new optics design for the OCT balloon imaging catheter. Different fr...